1. Field of the Invention
The present invention relates to a synchronous reluctance motor having a plurality of flux barriers, and more particularly, to a barrier type synchronous reluctance motor with an improved flux barrier that is capable of reducing torque ripple.
2. Description of the Background Art
Generally, two axes are used for controlling a motor: one is a D axis, that is, a boundary of a magnetic pole, and the other is a Q axis, that is, a center of the magnetic pole. The D axis has a high magnetic permeability, and the Q axis has a low magnetic permeability. A torque is generated due to an inductance difference between the D axis and the Q axis.
FIG. 1 is a plan view of a synchronous reluctance motor having flux barriers in accordance with the conventional art.
For the convenience of explanation, an example is taken where there are four poles and 24 stator slots.
As shown in FIG. 1, a stator 10 includes a body 11, a plurality of slots 13 and teeth 14 formed at the inner circumferential surface 12. A rotor 20 includes a body 21, a plurality of flux barriers 22 and a plurality of ribs.
As to the ribs, there are provided real ribs 23 (demonstrated by a circle), that is, the end portion of the flux barrier and virtual ribs 24 (demonstrated by `x`)., and the interval therebetween (including the actual rib 23 and the virtual rib 24) is the same as the pole pitch of the rotator slot.
The body 21 of the rotor 20 is made of magnetic substance, and the plurality of flux barriers 22 is made of a non-magnetic substance, which are radially arranged having the Q axis as a center and D axis as a boundary.
The operation of the synchronous reluctance motor having the flux barriers in accordance with the conventional art will now be described.
First, an inductance is generated by a current applied to the winding coil of the stator 10, and a torque is generated due to the inductance difference between the D axis and the Q axis owing to the flux barrier 22, and the rotor is rotated by the torque.
Torque ripple is generated due to the relative position of the flux barrier and the slot opening 13a of the stator 10. In other words, the torque ripple is generated due to a variation of the relative position of the slot opening 13a of the stator 20 and the rotor rib 23.
FIGS. 2 and 3 are plan views of the synchronous reluctance motor in the case where the rotor 20 is placed at specific positions when the motor of FIG. 1 is operated. FIG. 2 shows the case where the ribs 23 and 24 of the rotor 20 face the slot opening 13a of the stator 10, and FIG. 3 shows the opposite case to that of FIG. 2, that is, the case where the rib 23 of the rotor 20 faces the teeth 14 of the stator 10.
The magnetic resistances which develops in each of the first and the second cases are much different to each other, and due to the magnetic resistance, variations a substantial torque ripple is generated when the rotor 20 is rotated.
For that reason, the torque ripple is substantial when the number of the rotor ribs (the actual rib 23 and the virtual rib 24) is the same as that of the stator slots and when the interval between the ribs is the same as that of the slots.
Since the torque ripple is determined by the arrangement structure of the flux barrier, to reduce the torque ripple, various techniques for designing the structure of the flux barrier have been developed and disclosed, which will now be described.
FIG. 4 shows a flux barrier type synchronous reluctance motor having a plurality of flux barriers in accordance with a conventional art, in which the number of the real ribs of the flux barrier 22 is greater than that of the slots 13.
In this case, even though some of the ribs face the slots 13, since the remaining other ribs face the teeth 14, the average value of the overall magnetic resistance is lowered although the magnetic resistance at specific portions remains quite large.
However, as to this kind of rotor having the plurality of flux barriers, since the structure of the rotor is complicated, its mechanical strength is weakened as compared to a rotor having a few flux barriers, and its fabrication is also difficult. This problem becomes more serious as the size of the rotor is reduced.
FIG. 5 is a plan view of a flux barrier type synchronous reluctance motor where D and Q axes are mechanically unbalanced in accordance with the conventional art.
Unlike the D axis of a general motor, the D axis is slanted toward one side as much as a predetermined angle .theta., so that the D and Q axes are mechanically unbalanced.
With this structure, since the spacial intervals of the ribs of the flux barriers 22 and the intervals of the slots 13 are not the same, the torque ripple is reduced. However, since the D axis and the Q axis are unbalanced, it is difficult to judge electric D axis and Q axis when the motor is controlled, and the inductance values Ld and Lq according to the D and Q axes are influenced, causing a reduction of the overall torque output.